Amplifier



Aug. 13, 1929. v MILLER 1,724,498

AMPLIFIER Filed Nov. 21. 1921 dnren i-or John M. Miller 3 w ef ii'o 63 IPatented Aug. is, was.

JOHN M. MILLER, 01* .WASHINGTON, DISTRICT OF COLUMBIA.

AMPLIFIER.

Application filed November 21, 1921. Serial No. 516,682.

My invention relates generally to an amplifier, particularly one usingthree element vacuum tubes which serve to amplify alternating currentsor voltages of the'higher frequencies of those used in radio signaling,and particularly to a means for obtaining and passing on an amplifiedvoltage from one stage to another in a radio frequency amplifier, andone of the novel features thereof resides in the provision of anamplifier circuit of a character which permits a high amplificationat-short waves throughout a relatively wide band of frequencies andprovision for shifting the location of this band in the frequency scale.The advantage thus secured is high amplification without the necessityof critical adjustment.

Other features will appear in the following description, reference beinghad to the accompanying drawing, in which:

The figure is a diagrammatic viewof one form of my amplifying system,which is merely for the purpose of illustration and is not to beconsidered in any limiting sense.

Reference characters a and b indicate respectively the usual receivingantenna and the secondary receiving circuit. The mannor in which theenergy is collected from the radio signaling waves and the particulartype of collector constitute no part of the present invention.

In the well known manner, a radio signal produces voltage variationsbetween the grid 9 and filament f of the first amplifier tube cl. Thesevariations produce current variations in the circuit between thefilament f and the plate p. These current variations produce amplifiedvoltage variations across the inductance coil 6, which coil, as will behereinafter described, has a variable high in ductance and a lowcapacity. Such a coil offers a high impedance to these current varationsif their frequency is within a wide band of frequencies.

The voltage across the coil 6 is applied between the grid '9' and thefilament f of the second tube (Z; the condenser 0 serving to prevent thehigh positive voltage of the plate battery from acting on the grid ofthe second tube, but permitting the voltage variations to betransmitted.

, The high resistance indicated by reference character 7', maintains thegrid at approximately the potential of the negative terminal of thefilament, but does not reduce apfurther similar stages of amplificationmay be used, but in the particular circuit I have shown, the third tube(2 is represented in a well known manner as a detector tube, in whichcase the condenser 0 should be of suitable capacity, and resistance 1"of proper resistance and connected to the positive terminal of thefilament.

The plate circuit of the detector tube may contain the customarytelephone receivers t, as, shown, or the input circuit of an audiofrequency amplifier. In lieu of the detector tube any other means ofutilizing or detecting the amplified radio frequency oscillations may beemployed. The main feature of this invention over my copendingapplication Serial No. 452,- 542, filed March 15, 1921, and a divisionthereof resides in the employment of coils e and 6 having a suitablevariable high inductance and low capacity, and I prefer to provide forthis variable feature by winding these coils as a long single layersolenoid on a cylinder of small radius and from a fine insulated wire,but it is to be understood that my invention is not to be limited to anyparticular form of winding, as the essential point is that the windingbe of variable inductance and low capacity. High inductance is possiblethrough the very large number of turns obtainable with a fine, insulatedwire on a long cylinder, and low capacity is secured through thefineness of the wire, the small radius of the turns and the single layerof turns. In this construction the insulation of the wire is thenremovedfrom the outside surface of the wire along a line parallel to theaxis of each coil to permit the sliding contacts 8 ands to make contactat any point along the co1ls.

The coils are so wound that the number of turns in the plate circuit ofthe tube can be varied by adjustment of the members 8 and s. In effectthis varies the inductances of the coils. The inductances of the coilsin combination with the coil and tube capacities act as a parallelsystem of inductance and capacity, and because of the relative highinductance and low capacity this system has a high impedance forcurrents in the plate circuit of frequencies over a relatively broadband about the resonant frequency of the system, and hence, highamplification Will be secured over such a band of frequencies. Byshifting the contacts s and s the resonant frequency, and hence, theband of high amplification, can be shifted to longer or shorter wavelengths 'While still maintaining a broad band.

I prefer to mechanically connect the sliding contacts 8 and s in such amanner that they move together along the coils, so that only oneadjustment device is required. It is desirable to connect the slidingcontacts to the positive terminal of the plate battery so that thecontacts themselves, the unused turns of the coils e and e, and anyother conductors in contact at this point will be connected into theamplifier circuit at a point where the alternating voltage to ground islow, which tends to reduce the capacity of the coils.

To obtain the best results it is desirable to use vacuum tubes having alow capacity between the tube electrodes, in particular a low capacitybetween the grid and plate of the tube, since the tube capacities ineffect introduce a capacity which is in parallel with the capacity ofthe coil.

While I have illustrated two stages of amplification, it is obvious thatone stage or any number of stages may be employed. Further, theamplification secured by the means described may be utilized in variousways other than that shown.

Having fully described my invention What I claim as new is:

In a radio frequency amplifier including a plurality of vacuum tubes incascade, each of said tubes having grid-filament and platefilamentcircuits, the combination of a circuit of variable resonance in thegrid-fila ment circuit of the first vacuum tube, a coil of highinductance and low capacity in each of the plate-filament circuits ofthe remaining vacuum tubes, a high resistance in the grid-filamentcircuit of each of the remaining tubes, condensers connected between theplate of each tube and the grid of each succeeding tube and mean Withinthe platefilament circuit of each tube for adjusting the effectivelength of each of said coils to such a value that the coil acts as apure inductance with substantially no capacity through a relativelybroad band of frequencies both above and below the frequency to whichthe resonantcircuit in the gridfilament circuit of the first tube istuned.

JOHN M. MILLER.

